1. Field of Invention
The present invention relates to the production of iron. More specifically, the present invention relates to the simultaneous production of iron, coke, and, optionally, power.
2. Description of the Related Art
Accordingly to the World Steel Association, worldwide iron and steel production for 2009 stood at 1.22 billion metric tons. In terms of the ironmaking processes used, Modern Blast Furnace Ironmaking by M. Geerdes, H. Toxopeus and C. Van Der Vliet reports that blast furnaces account for approximately 60%, electric arc furnaces account for approximately 34%, and alternative ironmaking accounts for approximately 6% of the total iron and steel production. A description of current alternative ironmaking processes is found in the paper “Overview of Direct Reduction and Alternative Ironmaking Processes and Products,” presented by Joseph J. Poveromo, Ralph M. Smailer at the Association for Iron & Steel (AIST) specialty training conference, Scrap Substitutes & Alternative Ironmaking V (Baltimore, Md., Nov. 2-4, 2008). In the United States, for example, blast furnaces account for more than half (approximately 55%) of all crude steel produced. Blast furnaces dominate global iron and steel production, and one can expect that trend to continue for decades to come, especially in regions, such as Asia, lacking sufficient raw materials to sustain high production levels from electric arc furnace operations.
The blast furnace process uses a large, countercurrent, high temperature reactor to reduce iron oxides and melt the iron/steel product. A comprehensive description of modern blast furnace operating practices is found in Modern Blast Furnace Ironmaking. The majority of modern blast furnaces use coke as the primary reductant and iron ore pellets or sinter as the primary iron rich feedstock. In summary, a modern blast furnace is characterized by the use of pulverized coal injection, oxygen enrichment, high blast temperatures, proper raw material loading (burdening) equipment, high quality coke, proper feedstock preparation, water cooled panels, and fully instrumented process control. A large modern blast furnace can produce in excess of 10 000 metric tons of high quality hot metal each day.
Electric arc furnace processes have evolved rather quickly in comparison to blast furnace processes and technological advances over the past twenty years or so have allowed this technology to increase its proportion of total steel production. The electric arc furnace process is a melting process only. Very little reduction takes place during the electric arc furnace process. Accordingly, the electric arc furnace process is largely dependent on cost effective and reliable sources of iron. The predominant feedstock is scrap. The amount of trace contaminants in the scrap determines the quality of the feedstock. Feedstock quality dictates hot metal quality and hence steel quality. With recent technological advances and strict raw material controls, the electric arc furnace route has made in-roads into the higher priced flat products markets. A large modern electric arc furnace can produce in excess of 4000 metric tons of hot metal each day.
Because a blast furnace represents a huge capital expenditure, productivity, which is measured in metric tons of hot metal divided by the working volume of the blast furnace (THM/m3) is closely watched by steel producers. Productivity of a blast furnace is limited primarily by furnace permeability and gas distribution within the iron ore burden. Furnace permeability refers to the amount of gas (i.e., air, oxygen, products of combustion) that can be forced through the blast furnace by the high pressure turbo-blowers supplying hot blast air. Gas distribution refers to a more or less equal bulk gas flow up through the iron ore burden to provide sufficient contact time for reduction and melting.
The pressure drop (resistance to gas flow) in a blast furnace is estimated from the Ergun Equation:
                                          Δ            ⁢                                                  ⁢            P                    L                =                                                            (                                  1                  -                  ɛ                                )                            2                        ⁢            U            ⁢                                                  ⁢            μ                                              ɛ              3                        ⁢                                                            Φ                  2                                ⁡                                  (                  Dp                  )                                            2                                                          (        1        )            where ΔP is the pressure drop across the bed, L is the length of the bed, ε is the void fraction of the bed, U is the superficial fluid velocity of the fluid, μ is the dynamic viscosity of the fluid, Φ is the sphericity of the particles, and Dp is the equivalent spherical diameter of the particles. For all practical purposes, more gas means greater productivity.
The form of the feedstock has a significant effect on productivity. Feedstock forms that improve blast furnace productivity are generally preferred and command higher market values than the less desirable counterparts. The common forms of iron ore used in conventional and alternative ironmaking processes include fine ore, lump, pellet, and briquette. Fine ore, or fines, is defined as iron ore with the majority of individual particles having diameters measuring less than 4.75 mm (0.1875 in). Lump is defined as iron ore with the majority of individual particles having diameters measuring more than 4.75 mm (0.1875 in). Pellets are generally defined as shaped iron ore concentrate mixed with a binder and hardened with average diameters of in the range of approximately 9.55 mm to approximately 16.0 mm (0.375 in to 0.625 in). Briquettes are agglomerations of iron ores in blocks with exemplary lengths in the range of 50 mm to 140 mm (1.97 in to 5.5 in), widths in the range of 40 mm to 100 mm (1.6 in to 3.9 in), and thickness in the range of 20 mm to 50 mm (0.79 in to 1.97 in).
Compared to the larger diameters and irregular shapes of lumps or briquettes, pellets generally produce a higher resistance to gas flow in a blast furnace. Conversely, pellets generally exhibit higher reduction and melting rates because the smaller diameter of pellets equates to a larger specific surface area. For blast furnace operators, pellets represent a compromise between optimal furnace permeability and effective reduction and melting.
In addition to the conventional blast furnace and electric arc furnace process, a brief survey of select alternative ironmaking processes is presented. The prior art is replete with various attempts to co-mix iron ore fines and metallurgical coal together and then process the mixture in a coke oven to produce a material suitable for blast furnace operations. An example of such work is U.S. Pat. No. 3,427,148 issued to Peters, et. al (the '148 patent). Although the '148 patent discloses that reduction of the iron in the mixture can be achieved even at low temperatures in the range of 1000° C. to 1204° C. (1832° F. to 2200° F.), it is well known that the coke produced by such coal/iron blends is not suitable for blast furnace operations. The coke with partially reduced iron interspersed is small, weak, and highly susceptible to CO2 attack in modern blast furnace operations. Moreover, the primary product (coke) quality is so greatly diminished that this is not a viable process.
The Sheet Material Inserting METallization (SMIMET) study described in Production of direct reduced iron by a sheet material inserting metallization process, ISIJ International, Vol. 41 (2001), Supplement, pp. S13-S16 (Kamijo, C., Hoshi, M., et. al.), investigated the possibility of using a rotary hearth furnace for production of Direct Reduced Iron (DRI) without the need for special preparation (pelletizing) of the raw materials. The SMIMET study was a laboratory study to determine the efficacy of mixing fine coal (94% of the particles having diameters less than 125 μm), fine ore (both hematite and magnetite with 94% of the particles having diameters less than 125 μm), and water, forming a 10 mm sheet of raw material in a nickel or alumina container, and placing this material into an inert atmosphere (N2 or CO2) electrically heated furnace at 1300° C. (2372° F.). The results of the SMIMET process study showed that iron reduction is possible using the volatile material devolatilized directly from coal, reduction occurs at a fairly fast rate (i.e., 10 mm converted in 15 min), and showed that a high degree of metallization (% metallic Fe divided by % total Fe) could be achieved in as little as 15 minutes. While the study showed promising results, the SMIMET process requires a rotary hearth furnace with all of its ancillary equipment. A rotary hearth furnace is quite expensive to build and the associated fuel/power costs are also substantial. Applicability of the DRI product as a blast furnace feed material is unknown and cannot be assumed because the SMIMET study makes no mention of the size, shape, or strength of the product. To date, no commercialization of the SMIMET process is known to the present inventor.
The COal-based METallization (COMET) process developed by the Centre for Research in Metallurgy in Liege, Belgium is very similar to the SMIMET process previously described. Notable differences are that, rather than mixing the iron oxides and coal, these two raw materials are fed onto the rotary hearth in alternating layers. The product produced by the COMET process is flat slabs of sintered DRI that must be cut, screened, and cooled for product recovery. The sintered DRI product generated by the COMET process would be suitable for electric arc furnace steel making but not for blast furnace steel making or foundry operations. Like the SMIMET process, the COMET process would require significant capital investment plus auxiliary fuels (natural gas, coke oven gas or pulverized coal). The devolatilized coal (char) is a waste product of little to no economic value.
The High-Quality Iron Pebble (Hi-QIP) process described in U.S. Pat. No. 6,126,718 to Sawa, et. al. (the '718 patent), represents an advancement on the SMIMET and COMET processes in that it does operate at temperatures sufficiently high to produce a form of pig iron. The product of the Hi-QIP process is referred to as iron pebble. As with the SMIMET and COMET processes, the Hi-QIP process uses a rotary hearth furnace with its attendant high capital cost and supplementary fuel requirements in order to operate at temperatures adequate to melt the iron. The '718 patent discloses that the iron pebbles are suitable for both electric arc furnace and blast furnace operations. The article, “Hi-QIP, A New Ironmaking Process,” by Sawa, et al., in Iron and Steel Technology 87-94 (March 2008) describes problems associated with commercialization of this process. Most notably they cite the “furnace energy efficiency and reduction of fuel unit consumption.”
It is important to note that the SMIMET process, the COMET processes, and the Hi-QIP process all produce DRI, which is higher quality than pig iron but significantly more costly to produce. The higher production costs effectively limit the use of DRI to electric arc furnaces which require higher quality raw materials. Because a blast furnace can process lower quality and cost raw materials, an ironmaking process that further reduces the cost of the raw materials while maintaining the minimum raw material quality or increases the quality of the raw materials without increasing the raw material cost is critical.
Clearly, the fundamentals of reducing iron oxides into metallic iron are well established. An iron oxide in the presence of a reducing gas is heated to create the reducing reaction. In a laboratory setting, a process for reducing small quantities of most any iron oxide should be achievable if the practicalities necessary for economic sustainability in a commercial setting are ignored. In other words, a successful process developed and tested in a laboratory on small quantities of raw materials may prove the concept, yet fail to provide a solution to the real world problems faced by the iron and steel industry. Many proposed industrial technology development projects have historically failed due to scale up problems. Scale up problems occur in processes that have been proven at the bench scale level (laboratory scale), and even at the pilot plant level (nominally about 1% to 5% of full scale), but have failed at the full scale production level. In the very broadest of terms, a vast majority of these failures have occurred because the small scale process is a continuous process that cannot be sustained at an industrial level, the reactions occur in spherical or cylindrical reactors that are not reproducible at an industrial level, or the reduced scale reaction kinetics cannot be achieved in a full scale production facility.
A fundamental challenge is the production of sufficient quantities of quality raw materials to meet the demands of the iron and steel industry. In this context, sufficient quantities are measured in metric tons. In evaluating the solution, the efficiency and cost are significant factors. The cost includes the costs associated with obtaining and processing the raw material. A “successful” but energy inefficient laboratory reduction process is not a solution when scaled up to a commercial setting. In other words, if the operating costs of the scaled process offset the advantages of using a low cost raw material or if the scaled process cannot produce sufficient quantities of the iron/steel product in a timely fashion, it amounts to little more than theory with no practical application.